I have an alternative hypothesis: At least some people who say they don't like onion are basing that opinion entirely on raw onion and refuse to consider and/or haven't noticed cooked onion.

Go to restaurant, get served a house salad, bite into the purple thing, OH GOD WHAT IS THAT?

Alliums (which includes onions) are not uncommon food allergy so there's something in your theory

I didn't like onions in my childhood. I grew fond of them when approaching adulthood, particularly when I moved out and started properly cooking for myself and buying my own fast food. Now a sandwich without onion doesn't taste right.

So I think it's rather a matter of a particular configuration of tastebuds. Just like I grew out of liking overly sweet foods.